The genetic control, structure and assembly of the membrane-bound nitrate reductase complex is being studied in Escherichia coli. Conditional mutants (temperature sensitive and cold sensitive) are being isolated in an attempt to identify the structural genes of this complex. In addition, the conditional mutants are being utilized to establish the required order of expressions of the seven genes which control the formation of this complex. The addition of molybdenum is being studied in vivo and in vitro to inactive enzyme which accumulates in chl D mutants. Molybdenum activation is also being studied in cells containing inactive nitrate reductase in which tungstate has replaced molybdate.